I'm using an iPhone 3GS 16GB Os4.0. I got this phone from my friend so I don't have the receipt for it. I have all accessories, orginal box and the phone. And I just checked the serial numbers on Apple Wedsite about the warranty and I'm so glad it's still under warranty. But I'm afraid that if I have any problems with this phone,I can't bring it to the Apple Store to fix for free or take the replacement because I don't have the receipt with it and they're gonna ask me a lot of questions about my carrier At&t that I'm using. Is that truth? What should I do if I have any problems with this phone even it's still under warranty? Thank you so much for your help.